Coccidiosis is a disease caused by infection with one or more of the many species of coccidia, intracellular protozoal parasites of the subphylum Apicomplexa and the genus Eimeria. Poultry is defined herein as domesticated birds that serve as a source of eggs or meat and that include such commercially important kinds as chickens, turkeys, ducks, geese, guinea fowl, pheasants, pigeons and peafowl.
Coccidiosis in chickens is known to be caused by several different species of Eimeria, namely Eimeria acervulina, E. maxima, E. tenella, E. necatrix, E. brunetti, E. mitis, E. praecox, E. mivati and E. hagani. Some people, however, doubt the true existence of the last two species. Low level infection with any of these Eimeria species results in a protective immunity to reinfection.
The species do differ in their pathogenic effect on chickens, the type of chicken also playing a role; thus, a broiler chicken will be subjected to a great deal of damage by a parasite such as E. acervulina or E. maxima because these parasitise large portions of the small intestine, where food digestion plays a major role.
E. maxima is the most immunogenic of the species listed above, producing good natural protection following infection. There are, however, strain variations with little or no cross protection between strains.
During the life cycle, the Eimeria parasite passes through a number of stages. The life cycle begins when the chicken ingests the infectious stage, known as the sporulating oocyst, during ground feeding or by inhalation of dust. In the case of E. maxima, the oocyst is unusually large. The wall of the sporulated oocyst is ruptured by a combination of mechanical grinding action and chemical action in the gizzard and intestinal tract, resulting in the release of four sporocysts. The sporocysts pass into the duodenum where they are exposed to bile and digestive enzymes resulting in the release of an average of ten sporozoites per sporocyst.
The sporozoites are mobile and search for suitable host epithelium cells in order to penetrate and reproduce in them. Following infection of an epithelium cell, the parasite enters the schizont phase of its life cycle, producing from 8 to 16 to &gt;200 merozoites per schizont. Once released from the schizont, the merozoites are free to infect further epithelium cells. After from two to five of these asexual reproduction cycles, the intracellular merozoites grow into sexual forms known as the female or macrogametocyte and the male or microgametocyte. Following fertilization of the macrogametocyte by the microgametes released from the microgametocyte, a zygote is formed which creates a cyst wall about itself. The newly formed oocyst is passed out of the infected chicken with the droppings.
With the correct environmental conditions of temperature and humidity and sufficient oxygen in the air, the oocyst will sporulate into the infectious stage, ready to infect a new host and thereby spreading the disease. Thus no intermediate host is required for transfer of the parasite from bird to bird.
The result of the Eimeria parasite infecting the digestive tract of a chicken may be a reduction in weight gain, decreased feed conversion, cessation of egg production and, in many cases, death. The increase in intensive production of poultry has been accompanied by severe losses due to this parasite; indeed, coccidiosis has become the most economically important parasitic disease. In the Netherlands, the losses that poultry farmers suffer every year run into millions of guilders; in 1986 the loss was about 13 million guilders. In the same year, a loss of 300 million dollars was suffered in the United States.
In the past, several methods have been used in attempts to control coccidiosis. Prior to the advent of chemotherapeutic agents, improved sanitation using disinfectants, together with the mechanical removal of litter, was the main method employed; sufficient oocysts, however, usually remained to transmit the disease.
The introduction of coccidiostatic agents in the feed or drinking water, in addition to good management, resulted in some success at disease control. Such agents have been found to suffer from a drop in effectiveness over the years, due partly to the development of drug resistant strains of coccidia. Furthermore, several chemotherapeutic agents have been found to leave residues in the meat, making it unsuitable for consumption.
Attempts have been made to control the disease immunologically by administering to chickens a live vaccine comprising oocysts from all seven species of Eimeria, the oocysts administered being from precocious lines. Such precocious lines are obtained by inoculating chickens with a wild population of an Eimeria species and collecting the very first parasites that are excreted as a result of the infection. The collected parasites are put back into chickens and the cycle is repeated several times. Eventually a precocious line of parasite is produced which has fewer cycles of asexual reproduction in the gut. Thus such lines retain their immunogenicity, whilst producing fewer parasites in the gut with less consequential damage being caused to the host chicken. The disadvantage of this type of vaccine is that it is expensive to produce because of the necessity of producing it in live chickens and its lower reproductive potential.
The advent of genetic engineering has provided new methods for producing effective vaccines. Using these methods, the DNA coding for the antigenic proteins of some pathogenic microorganisms has been cloned into such host microorganisms as Escherichia coli, with the result that the protein has been expressed at sufficiently high levels such that it can be incorporated into a vaccine. The advantage of proteins produced in this way is that they are noninfectious and are relatively cheap to produce. In this way, vaccines have been prepared against a number of viruses such as hepatitis, herpes simplex and foot and mouth disease.
Attempts have been made to genetically engineer a coccidiosis vaccine. European patent application No. 337 589 describes the isolation of a Group B Eimeria tenella protein and its insertion into a novel expression vector which, in turn, has been used to transform appropriate hosts. Patent Cooperation Treaty Application WO 92/04461 describes the construction of a microorganism that produces an antigenic protein using either the "mRNA route" or the "nuclear DNA route". In this way, certain antigens from E. tenella and E. maxima were prepared and sequenced. Taking this type of route to prepare antigens for incorporation into a vaccine relies only upon selecting antigens which could induce antibodies in an heterologous species. This approach does not necessarily end up with selecting the most protective antigen.